The present invention relates to a light-emitting element such as a light-emitting diode, to an array of light-emitting elements, and to a fabrication method for the invented light-emitting element and array.
Light-emitting diodes or LEDs are widely employed as display and indicator devices, because they emit bright light and can be driven at low voltages by simple circuitry. LED arrays are used as light sources in electrophotographic printers.
A conventional LED or LED array is fabricated by diffusing a p-type impurity into an n-type compound semiconductor substrate, forming a pn junction at the diffusion front. Either vapor-phase or solid-phase diffusion can be employed. During the diffusion process, the substrate is heated to a high temperature, so while impurity atoms diffuse into the substrate, atoms also diffuse outward from the substrate, leaving crystal defects in the substrate. A large number of such crystal defects form near the surface of the substrate.
During operation, a forward voltage is applied, injecting minority carriers across the pn junction. Some of the injected minority carriers recombine with majority carriers by a radiative process, generating photons. Some of the generated photons reach the surface of the substrate and are emitted as light. Other photons are absorbed within the substrate.
Although recombination is most likely to occur near the pn junction, a certain fraction of the electrons injected into the p-type diffusion region reach the surface vicinity before recombining with holes. If the recombination is radiative, then since the generated photons have only a short distance to travel to the surface, their probability of being absorbed is low, and their probability of being emitted from the surface is high. Recombination near the surface is therefore a potentially important source of emitted light.
In conventional LEDs, however, recombination near the surface has a high probability of not being radiative. This is partly because of a surface potential with a deep energy level that promotes nonradiative recombination. However, another major factor is that the many crystal defects near the surface act as nonradiative recombination centers.